Thermal IR sensors and methods of measuring IR radiation by means of semiconductor circuits, in particular by making use of MEMS structure suspended in a cavity, and comprising a temperature sensor, are well known in the art.
Most IR sensors transfer an incoming IR signal into a temperature increase of a thermally isolated structure (referred to herein as “absorber”) arranged for absorbing the IR radiation. The absorber is typically a so called membrane or diaphragm suspended in a sealed cavity by means of a suspension structure (e.g. long and thin beams). It's noted that the absorber feature is not limited to a dedicated absorbing layer, but is seen as the general feature of the detector to collect IR radiation by means of absorption. The more IR-power the isolated structure receives, the higher the temperature of the absorber will be with respect to the bulk (substrate and cap). For each amount of IR radiation, there is an equilibrium temperature at which the heating-up due to the incident IR power equals the heat loss from the absorber to the surrounding substrate and cap via heat conduction, heat convection and heat radiation. The temperature increase of the absorber is thus an indication of the amount of incident IR radiation, and is typically measured by means of a resistor with a high temperature dependence (bolometer) or by means of a series of thermocouples (thermopile). In this document the heat absorber with temperature sensor is referred to as a “pixel”.
For good sensitivity of the pixel, the temperature output signal would preferably be as large as possible for a given amount of IR-radiation power. The sensitivity of such a pixel is determined by three physically different gain factors: The first factor is determined by transmission, absorption and reflection of the IR light through the lid or cap onto the absorber. The second gain factor is determined by the thermal heat resistance between the absorber and the bulk of the device. This second factor especially depends on the heat resistance through the suspension structure (e.g. beams) of the thermally isolated structure and the heat resistance through the surrounding gas. The third factor is determined by the thermometer which is typically a resistor with high temperature dependence or a series of thermocouples (thermopile).
The stability and linearity of such a sensor clearly depends on the stability and linearity of the different gain factors, and often compromises have to be made between signal amplification and amplification stability. One of the most difficult parameters to control is the heat conduction from the absorber through the gas that surrounds the absorber.
The problem is that the sensitivity of infrared detectors can change over time due a potential change over time of the heat conduction from the absorber through the gas that surrounds the absorber. This could be induced by e.g. any small outgassing process, any small hermeticity leakage or any small permeability which would result in a change of the internal system pressure. This would finally lead to an error in the output signal.